In industry, marking devices such as strip chart recorders, metering chart recorders and printers are used to record various physical properties over time, depth or other indices. One such industry is the oil and gas industry. In wireline well logging, one or more tools are connected to a power and data transmission cable or "wireline" and lowered into the well borehole to obtain measurements of geophysical properties for the area surrounding the borehole. The wireline supports the tools as they are lowered into the borehole, supplies power to the tools and provides a communication medium to send signals to the tools and receive data from the tools. Data from the tools is directed by the wireline to data acquisition and processing equipment at the surface. The data acquisition and processing equipment compiles the data from the tools into a "log," a plot which presents the geophysical information concerning the formations encountered by the well, frequently by depth. U.S. Pat. No. 5,051,962 (incorporated by reference) describes such a well logging system controlled by a general purpose computer programmed for real time operation. The log is translated from analog or digital readings into physical form by a marking device such as a printer. Examples of such printers include Schlumberger's proprietary TGRP, CGRP and CIDP models, as well as commercially available models such as Printex model 820 DL and Gulton model 295. Logs can also be used to evaluate current production or to inspect the integrity of production equipment in a producing well. In any case, the information gathered during the logging operation is generally presented on the log by depth, but may also be presented by time, or any other index by which multiple physical entries are recorded.
Professionals in the industry such as geologists and geophysicists, often use these logs to update their understanding of the producing formations and geology. Standard practice is to compare logs from different wells by analyzing them side by side to assess at what depths each well encountered particular formations and how the formations differ from well to well. This comparison may be accomplished by visually comparing the printed logs side by side. It is therefore important to maintain an accurate depth scale, which lies along the longitudinal or "long" axis of the log, so that comparisons between wells can readily be made. While the transverse scale is relatively short, typically less than a foot, the longitudinal scale in a log for a deep well may be over twenty feet long. Other important comparisons may include time variations of information on the same well or comparing the logging curves on the same well which measure different geological characteristics.
The print medium is commonly some type of paper or film. The supply of print medium may be in the form of rolls, fan folds or any convenient configuration. Since the medium on which the log is printed is typically advanced by a paper transport system with rollers within a printer, the thickness of the medium can affect how far the printer advances the medium for a given angular rotation of the roller(s). This will affect whether the longitudinal scale of the log is printed accurately, i.e., to the proper physical scale. Even variation in the moisture content of paper media can produce significant variation in the longitudinal scale. In most printers, the transverse scale is not subject to these variations, because the printing heads either are fixed or are otherwise constrained. In addition, a paper transport system that uses sprocket gears instead of rollers and media with sprocket holes is not subject to these variations, at least not to any great extent.
English (non-metric) depth scales commonly used in the industry include one (1) inch of log per hundred feet of well depth (a "one inch log") and five (5) inches per hundred feet of well depth (a "five inch log"). While attempting to print a five inch log, printer and media variations may instead result in a log with a scale printed as if it were five and one-sixteenth inches per hundred feet. The numerical depths printed will be correct with respect to the depths of the formations encountered, but comparisons with other logs will be difficult to make. Wells are typically logged from the total depth up to the bottom of the deepest casing installed in the well. As an example, if casing is set at a depth of 5000 feet in a 10,000 foot well, the log incorrectly printed with a five and one-sixteenth inch scale would be "off" by 3.125 inches or the equivalent of over 62 feet by the time it reached the casing, when compared to a neighboring well with a properly printed five inch log. And a 300 foot long formation in the improperly printed log would appear as if it were over 318 feet long, when compared to formations shown in the properly printed log.
If an industry professional is attempting to compare two logs, the longitudinal scales of which are equally but antithetically inaccurate, the difficulty is compounded. If the five and one-sixteenth log is being compared, for instance, to a log which was improperly printed with 100 feet to every four and fifteen-sixteenths inches of log, a 300 foot long formation in the five and one-sixteenth inch log will appear like a 337 foot long formation in the four and fifteen-sixteenth log.
Some markers, strip chart recorders and printers have ways of compensating for such variations in their hardware, but others do not. The hardware compensation methods vary from printer to printer and use of these methods can be very time consuming. For example, the TGRP model printer uses a thumbwheel for incremental corrections. The Gulton model 295 uses a complicated procedure involving binary fractions and dual in-line package ("DIP") switches. The CIDP model has no way to compensate for media variations. Most methods also require the need for external standards for calibration, such as a ruler. Wellsites are frequently remote and usually relatively isolated. The need for an external standard can be inconvenient when logging at wellsites where such a standard may easily be misplaced or lost and replacement may be difficult or even impossible to accomplish in a timely manner. Also, if one determined a way to compensate for variations in a particular printer and/or media, it would be useful to retain that information for future use.
Therefore, a need has arisen for a convenient and uniform longitudinal axis calibration procedure which is applicable to all marking devices, such as strip chart recorders, metering chart recorders and printers. A further need exists for a calibration procedure which can be used with all types of print media. A further need exists for a calibration procedure which does not require an external standard. A further need exists to retain calibration information for future use.